Tesla Solar Roof estimate questions

[cobra]

I'm learning a lot here on the Tesla Energy section! So help me understand Tesla's estimate tool. I think they are not showing some savings for PowerWall, for example, but also, I'm not sure how they calculate the basic 25yr savings. In the attached image, I picked a simple solar roof in the Philadelphia, PA suburbs and claimed $300/mo electric usage. I've played with the electric bill amount some and it always says savings is $172/mo for a system of this size. I've seen some estimate that the PA average electric rate is about $0.12 state wide, while we are at $0.16 straight rate at present. I'm trying to figure out how they come up with a $19,888 savings number for the system. It seems to me given they are showing average savings of $172/mo overall, that it ought to be $172 x 12 mo x 25 yrs, but that is $51,600. So not even in the ballpark. I tried subtracting off the cost of the roof and solar roof components and come closer, but the numbers still don't match. Has anyone figured out what Tesla is doing here?

I'm really trying to figure out why I can't get $300 of electricity out of the estimator. I assume without Powerwalls that nighttime use is payed from the grid. So I add Powerwalls and the savings stays the same! Only ever shows $172 saved, no matter how I change the configuration. (But savings goes down for the 25year period... )

 

[wjgjr]

For the $172/$300 numbers, did you increase the size of the roof in the estimator? The screenshot shows a 2,671 sq. ft. roof, and when I put that into an address, it maxes out at the 16.1 kW size you were given. If you click the "Edit" button and change it to 3,000 or 4,000 sq. ft., you should be able to get a bigger system size and thus a larger reduction on the monthly bill.

As to the 25-year savings, I assume that it is based on comparing the final system price after incentives ($46k) with the estimated total electric savings with Tesla's assumptions about how electric prices will change (presumably increase) over that time. A 2% increase annually, with no other adjustments, would get you to those savings. (I do not know how Tesla actually calculates this - for example, whether they are discounting future savings due to inflation and panel degradation, for example.)

In fact, I just ran the numbers in MD, and applying a 2% annual increase on the electric bill savings here also gets me within a few dollars of the Tesla estimate.

 

[tfan2018]

It isn't seem Tesla is factoring any cost savings from adding a power wall. It shows $7,770 for me to add a pw, but that exact amount is deducted from the 25 year savings.

 

[wjgjr]

It isn't seem Tesla is factoring any cost savings from adding a power wall. It shows $7,770 for me to add a pw, but that exact amount is deducted from the 25 year savings.

I do not know whether or if Tesla provides and savings estimates for PWs. (If I were Tesla, I probably wouldn't, but I would also exclude it from the savings calculation in general, making the calculation specific to savings from the solar panels/roof.) The problem, as has been discussed in more detail elsewhere, is that the savings, if any, depend on your location and on how you use your PWs. I am not sure your location, but here, we have full net metering and no TOU rates, so there are no powerwall savings. Other places have different rates, rules and programs that may include savings, but the differences may vary by jurisdiction and rate plan, and I'm not sure it would be easy for Tesla to communicate all of that. Perhaps they could for the grid stabilization programs that they are running in parts of New England, but a lot of other things are very local and specific to the customer's use patterns and plans for the PWs.

 

[cobra]

For the $172/$300 numbers, did you increase the size of the roof in the estimator? The screenshot shows a 2,671 sq. ft. roof, and when I put that into an address, it maxes out at the 16.1 kW size you were given. If you click the "Edit" button and change it to 3,000 or 4,000 sq. ft., you should be able to get a bigger system size and thus a larger reduction on the monthly bill.

As to the 25-year savings, I assume that it is based on comparing the final system price after incentives ($46k) with the estimated total electric savings with Tesla's assumptions about how electric prices will change (presumably increase) over that time. A 2% increase annually, with no other adjustments, would get you to those savings. (I do not know how Tesla actually calculates this - for example, whether they are discounting future savings due to inflation and panel degradation, for example.)

In fact, I just ran the numbers in MD, and applying a 2% annual increase on the electric bill savings here also gets me within a few dollars of the Tesla estimate.

That's a good observation... I had not increased roof size. Main reason was because I saw the same savings number for solar panel estimate. I thought the energy density was a little lower for solar roof, so wrongly assumed, it seems, that something else was going on. You are correct, going to 3500 sqft roof raises the amount saved.

I had not tried adjusting the electric rate over time but that makes sense.

I do not know whether or if Tesla provides and savings estimates for PWs. (If I were Tesla, I probably wouldn't, but I would also exclude it from the savings calculation in general, making the calculation specific to savings from the solar panels/roof.) The problem, as has been discussed in more detail elsewhere, is that the savings, if any, depend on your location and on how you use your PWs. I am not sure your location, but here, we have full net metering and no TOU rates, so there are no powerwall savings. Other places have different rates, rules and programs that may include savings, but the differences may vary by jurisdiction and rate plan, and I'm not sure it would be easy for Tesla to communicate all of that. Perhaps they could for the grid stabilization programs that they are running in parts of New England, but a lot of other things are very local and specific to the customer's use patterns and plans for the PWs.

I live in a rather unusual medium size town that does not have Net Metering and looks like they disallow power flow back to the grid completely. Given that, Powerwall use at night looks like the only option for reducing the non-generation portion of grid usage (using Tesla options, that is.) Thanks for confirming they do not include it in energy savings. That's what I was starting to suspect. I'll have to work up my own numbers.

 

[wjgjr]

I live in a rather unusual medium size town that does not have Net Metering and looks like they disallow power flow back to the grid completely. Given that, Powerwall use at night looks like the only option for reducing the non-generation portion of grid usage (using Tesla options, that is.) Thanks for confirming they do not include it in energy savings. That's what I was starting to suspect. I'll have to work up my own numbers.

How would that even work if it disallows power flow to the grid completely? That would mean you would need PWs and for solar to stop when PWs are full, or some other solution that curtails solar to match home use (and would still probably have some flow.) Is it they disallow it or they just pay you nothing (which I recall a few others mentioning with their utilities.)

 

[tfan2018]

I do not know whether or if Tesla provides and savings estimates for PWs. (If I were Tesla, I probably wouldn't, but I would also exclude it from the savings calculation in general, making the calculation specific to savings from the solar panels/roof.) The problem, as has been discussed in more detail elsewhere, is that the savings, if any, depend on your location and on how you use your PWs. I am not sure your location, but here, we have full net metering and no TOU rates, so there are no powerwall savings. Other places have different rates, rules and programs that may include savings, but the differences may vary by jurisdiction and rate plan, and I'm not sure it would be easy for Tesla to communicate all of that. Perhaps they could for the grid stabilization programs that they are running in parts of New England, but a lot of other things are very local and specific to the customer's use patterns and plans for the PWs.

that makes sense. I also feel better a lot better about spending $ for the powerwall now that I see the 25 years savings doesn't include savings from TOU powerwall arbitrage.

 

[cobra]

How would that even work if it disallows power flow to the grid completely? That would mean you would need PWs and for solar to stop when PWs are full, or some other solution that curtails solar to match home use (and would still probably have some flow.) Is it they disallow it or they just pay you nothing (which I recall a few others mentioning with their utilities.)

Yes, probably. I was reading here that the new Tesla inverter can throttle PV generation, so I expect that will be the case. Permit application says something like -customer may not backfeed power in to the grid. Cave man stuff, right?

 

[cobra]

After looking some more, I realized I could calculate Tesla's first year estimate of energy cost savings in my state from the annual production number and their $172/mo savings. It comes out to $0.122/kWh which is pretty close to state average numbers I have seen here in PA. Since my local rate is around $0.16/kWh I will do somewhat better. I need to do some more estimates with daily numbers to see how solar and solar + PW would work.